res_pjsip.c: Rework endpt_send_request() req_wrapper code.

* Don't hold the req_wrapper lock too long in endpt_send_request().  We
could block the PJSIP monitor thread if the timeout timer expires.
sip_get_tpselector_from_endpoint() does a sorcery access that could take
awhile accessing a database.  pjsip_endpt_send_request() might take awhile
if selecting a transport.

* Shorten the time that the req_wrapper lock is held in the callback
functions.

* Simplify endpt_send_request() req_wrapper->timeout code.

* Removed some redundant req_wrapper->timeout_timer->id assignments.

Change-Id: I3195e3a8e0207bb8e7f49060ad2742cf21a6e4c9